1. Field of the Invention
The present invention relates to an apparatus and method for detecting fluorescence or light scatter, which may relate to the presence or absence of an analyte, in a liquid sample. In particular, the invention relates to a dual optical fiber probe in which the spatial configuration of the optical fibers is arranged to define a finite interrogation volume independent of the volume of the liquid sample. The interrogation volume can be visualized as the intersection of the two light cones emanating from the first and second optical fibers having a determined spatial configuration when each optical fiber transmits visible light. The size of the interrogation volume is determined by the spatial orientation and optical parameters of the dual optical fibers, preferably creating an interrogation volume of less than 10.sup.-5 cubic centimeters. The invention is particularly useful for clinical laboratory applications such as blood typing.
2. State of the Art
Cells and other particles in the range of 0.05 to 100 microns suspended in a liquid medium have been individually detected by streaming the liquid to provide a fine stream or to create very small droplets so as to provide a detection volume and monitoring of the light scattered by the particles, including Rayleigh and Raman light scattering, or that emitted from the particles such as by fluorescence. The basic principle of light scattering from a small volume is used in flow cytometers. Commercial instruments utilizing light scattering and fluorescence for particle detection are available.
PCT Patent Application Publication No. WO 85/05680, published Dec. 19, 1985, describes an optical system for use in flow cytometers that include multiple optical fibers disposed about and on an orifice plate used to define the flow stream.
Briggs (U.S. Pat. No. 4,564,598) describes principles for the detection of particles that are not confined to small volumes. As described therein, a small detection volume is achieved by illuminating the bulk suspension with a very narrow beam of light, e.g. 10-100 microns, delivered through an optical fiber immersed in the fluid and collecting the light scattered at the tip of the fiber by allowing it to pass back through the fiber past a beam splitter. Because the same fiber is used for illuminating the particles as well as collecting the emitted light, the method is generally useful only where the emitted light is wavelength shifted as in fluorescence. Under these conditions, internal scatter of the fiber can be largely filtered out to permit detection of the signal. However, even when detecting fluorescence, the background scatter limits sensitivity. While in principle multiple wavelengths can be monitored, multiple splitters must be used which further decrease sensitivity and also increases the cost and complexity of the system.
Other devices and apparatus have been described for the detection and measurement of scattered and emitted light from particles. United Kingdom Patent No. 1,506,017 describes a fluorometric system including two optical fibers that transmit and receive electromagnetic radiation to and from the sample-coated surface of a macrobody (typically a sphere on the order of 5-20 millimeters in diameter). The optical fibers are preferably disposed at a small angle, less than thirty degrees, relative to each other. The fibers are not utilized to define a volume in a liquid sample from which the presence or absence of an event will be monitored.
Japanese Patent Application No. 84230306, filed Nov. 2, 1984 (Publication No. 61110033, dated May 28, 1986) describes the use of a plurality of optical fibers to measure particle agglutination.
U.S. Pat. No. 4,564,598 (Briggs), noted previously, describes an optical fiber probe having a single optical fiber, that both supplies the incident radiation and receives the excitation signal, to measure fluorescent signal in a fluid sample. U.S. Pat. No. 4,537,861, (Elings et al) describes an immunoassay technique in which labelled, bound ligand-antiligand complexes are caused to reside in a predetermined spatial pattern that then can be scanned to detect a signal level relative to background signal. European Patent Publication No. 0175545, published Mar. 26, 1986 (Application No. 85306450.9, filed Nov. 11, 1985) describes a method for particle detection that autocorrelates the intensity of an electromagnetic signal over a nonzero interval, the duration of which is short compared to the mean duration of the fluctuations. Additional documents describing particle detection methods are U.S. Pat. No. 4,421,860 (Elings et al) and U.S. Pat. No. 4,560,881 (Briggs) and references described and listed therein.